1. Field of the Invention
This invention relates generally to bandpass filters and more particularly to solid state microwave bandpass filters which utilize surface acoustic waves in a piezoelectric medium.
2. Description of the Prior Art
Surface acoustic wave (SAW) devices are often employed as filters or resonators in high frequency applications.
The advantages of using SAW devices over other frequency control methods such LC circuits, coaxial delay lines, or metal cavity resonators are high Q, low series resistance, small size and good frequency-temperature stability. SAW resonators also possess advantages over bulk acoustic wave resonators because the latter must be cut very thin to achieve high frequencies and are consequently quite fragile.
Typically, a SAW device contains a substrate of piezoelectric material such as quartz, lithium niobate, zinc oxide or cadmium sulfide. Input and output transducers are formed upon the substrate. The transducers convert input electrical signals to surface acoustic waves (SAWs) propagating upon the surface of the substrate and then reconvert the acoustic energy to an electric output signal. The input and output transducers are frequently configured as interdigital electrode fingers which extend from pairs of transducer pads. Interdigital transducers may be formed by depositing a thin film of electrically conductive material upon a piezoelectric substrate.
Alternating electrical potential coupled to the input interdigital transducers induces mechanical stresses in the piezoelectric substrate. The resulting strains propagate away from the input transducer along the surface of the substrate in the form of surface acoustic waves. These propagating surface waves arrive at the output in interdigital transducer where they are reconverted to electrical signals.
An article pertinent to the understanding to the present invention is: P. M. Naraine and C. K. Campbell, "Wideband Linear Phase SAW Filters Using Apodized Slanted Finger Transducers", Proc. IEEE Ultrasonics Symposium, October 1983, pp. 113-116. The Naraine et al. article discusses a method for designing wide band linear phase SAW filters using apodized slanted finger transducers. Slanted finger transducer geometries have all of the transducer fingers positioned along lines which emanate from a single focal point. The use of slanted finger geometeries on both the inut and output transducers permits the transduction of a wide range of surface acoustic wavelengths from input to output transducer, and thus, provides an electrical filter with a wide frequency passband. High frequency components are transduced across the side of the substrate nearest the input and output focal points where the finger-to-finger distance is least. Low frequency components are transduced across the opposite side of the substrate where the finger-to-finger distance is greatest. The Naraine article states that for filters employing slanted finger transducer geometries, "both theory and experiment indicated an inherent negative slope of the amplitude response with increasing frequency, which could be as large as approximately 5 dB for the 50% bandwidth case." op. cit. p. 113. Naraine's article describes a method of flattening the amplitude response curve of a slanted finger filter by utilizing finger apodization. Apodization is a technique in which the length of individual transducer fingers is selectively adjusted so that the overlap between fingers of opposite polarities changes along the path traveled by the surface acoustic wave. Naraine discloses the results of a computer optimization program which is used to obtain a transducer finger apodization pattern which modifies the slanted finger geometry. The result is a flatter amplitude response curve. However, the effect of the Naraine apodization technique is to achieve a flat passband by reducing the coupling of low frequency components, thus reducing the amplitude of the low frequency portion of the amplitude response curve.
The overall performance of a slant-finger SAW filter would be enhanced if the amplitude of the high frequency portion of the response curve were increased. The result would be an amplitude response curve with the desired flat plateau and a greater overall amplitude. The present invention achieves a flat amplitude response curve by increasing the coupling of high frequency components instead of decreasing the coupling of low frequency components. Thus the present invention achieves a smooth, flat passband without sacrificing low frequency signal amplitude.